A Theoretical Kinetic Study of the Reaction between CH3OȮ and ṄH2

In NH3/CH4 combustion, the oxidation product CH3OȮ derived from ĊH3 radicals can react with abundant ṄH2 radicals, and the CH3OȮ + ṄH2 ↔ CH3Ȯ + H2NȮ reaction can significantly increase reactivity and influence the model-predicted ignition delay times of NH3/CH4 mixtures at low to intermediate temperatures. In this work, the detailed reaction kinetics between CH3OȮ and ṄH2 and the subsequent unimolecular decomposition processes are investigated. High-level ab initio calculations are performed for all species using the ANL0 method. Variable reaction coordinate transition state theory (VRC-TST) is employed to treat the barrierless radical-radical reaction channels, based on directly sampled interaction energies calculated at the CASPT2-F12/cc-pVTZ-F12 level of theory, combined with one-dimensional (1-D) geometry and basis set corrections. To properly describe the electronic structures of different reaction channels, a (2e,2o) active space is employed for the entrance channel of CH3OȮ + ṄH2 ↔ CH3OONH2, whereas a larger (6e,5o) active space is used for the dissociation reaction channels of CH3OONH2 ↔ CH3Ȯ + H2NȮ and CH3ONH2O ↔ CH3Ȯ + H2NȮ. The reactive fluxes obtained are then coupled with Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) calculations to calculate temperature- and pressure-dependent rate constants. The results indicate that the formation of CH3Ȯ + H2NȮ is the dominant reaction channel. At the high-pressure limit (HPL), the dominant pathways for CH3OONH2 are isomerization to CH3ONH2O and dissociation to CH3Ȯ + H2NȮ. The CH3ONH2O isomer further decomposes to CH3Ȯ + H2NȮ and becomes the dominant pathway at T > 1100 K. This study provides valuable kinetic parameters for CH3OȮ + ṄH2, which is one of the key reactions in NH3/CH4 combustion in the temperature range 600-1500 K.